Calculate pre-plate and pre-anodize thread dimensions for internal and external threads. Coating buildup changes thread size — machine to allowance dimensions before plating so the final threaded feature meets tolerance after coating.
Coating thickness is per side (one surface). Diameter changes by 2× this value.
Results
Electroless NickelThickness per side: 0.001" (25µm) · Diameter change: 0.002"
PRE-PLATE (Machine To)
Major ⌀:—
Pitch ⌀:—
Minor ⌀:—
Class:—
POST-PLATE (Final Size)
Major ⌀:—
Pitch ⌀:—
Minor ⌀:—
Class:—
Coating Per Side
—
single surface
Dia Change (2× side)
—
on diameter
Pre-Plate Major ⌀
—
machine to this
Post-Plate Major ⌀
—
final dimension
Rec. Pre-Plate Class
—
thread tolerance class
µm Per Side
—
coating thickness
ReadySelect thread size, type, and coating process, then click Calculate.
Detailed Summary
Thread
Thread size
—
Thread type
—
Nominal major diameter
—
Pitch
—
Coating
Coating process
—
Thickness per side
—
Thickness per side (µm)
—
Diameter change (2× side)
—
Pre-Plate Dimensions (Machine To)
Major diameter
—
Pitch diameter
—
Minor diameter
—
Recommended class
—
Post-Plate Dimensions (Final)
Major diameter
—
Pitch diameter
—
Minor diameter
—
Final thread class
—
How Plating Changes Thread Dimensions
Every coating process adds material to the thread surface. For a 0.001" per side coating, the major diameter grows by 0.002", the pitch diameter grows by 0.002", and the minor diameter grows by 0.002" — all surfaces are coated equally. On an external thread (bolt), this growth makes the thread larger and tighter — potentially making it impossible to assemble without re-tapping the nut. On an internal thread (nut), the coating shrinks the bore, reducing clearance. The solution is to machine the pre-plate thread to an oversize (external) or undersize (internal) condition so the coating brings it back to the nominal thread class.
1 External Thread (Bolt) Allowance
External threads grow larger with coating. Machine to an undersize condition before plating. The amount of undersize equals the coating thickness per side (applied to all diameters on both sides = 2× thickness change per diameter).
External Thread (bolt/stud/pin):
Coating adds to all surfaces
Major Dia Change: +2 × coating_side
Pitch Dia Change: +2 × coating_side
Minor Dia Change: +2 × coating_side
Pre-plate major = final_major − 2×t
Pre-plate pitch = final_pitch − 2×t
Pre-plate minor = final_minor − 2×t
(t = coating thickness per side)
Example: ½-13 external, 0.001" EN:
Final major = 0.5000"
Pre-plate major = 0.5000 − 0.002
= 0.4980"
Machine to 0.4980" before plating
2 Internal Thread (Nut) Allowance
Internal threads shrink with coating — the bore gets smaller as material is added to the thread walls. Machine to an oversize condition (larger bore) before plating so the coating brings it back to nominal thread class.
Internal Thread (nut/tapped hole):
Coating fills into the thread
Major Dia Change: −2 × coating_side
Pitch Dia Change: −2 × coating_side
Minor Dia Change: −2 × coating_side
Pre-plate major = final_major + 2×t
Pre-plate pitch = final_pitch + 2×t
Pre-plate minor = final_minor + 2×t
Example: ½-13 internal, 0.001" EN:
Final pitch dia = 0.4500"
Pre-plate pitch = 0.4500 + 0.002
= 0.4520"
Tap to produce 0.4520" pitch dia
3 Thread Class Selection
ASME B1.1 defines thread classes 1A/1B (loose), 2A/2B (standard), and 3A/3B (close). For plated fasteners, the pre-plate thread is typically machined one class looser than specified. The coating then fills the clearance to arrive at the correct class.
Common pre-plate class selection:
Final class → Pre-plate class:
3A/3B → machine to 2A/2B
2A/2B → machine to 1A/1B or 2A/2B
(depending on coating thickness)
1A/1B → special — consult plater
For light coatings (<0.0002" / 5µm):
No class change needed — use same
class with allowance at upper limit
For thick coatings (>0.001" / 25µm):
Always go one class looser
Verify with thread gauges
after plating
4 Anodize vs. Electroplate
Anodizing (for aluminum) is unique — approximately 50% of the anodize layer grows into the base metal and 50% grows outward. Type II anodize builds up ~0.0001" outward per side. Type III (hard anodize) builds up much more and significantly affects thread dimensions.
Anodize layer behavior:
~50% grows INTO base metal
~50% grows OUTWARD
Net outward growth (per side):
Type I (chromic): ~0.00005" outward
Type II (sulfuric): ~0.00010" outward
Type III (hard): ~0.00050–0.00100"
For threads: use OUTWARD growth value
(the dimension that affects fit)
Example: Type III hard anodize, 0.001"
total layer, 0.0005" outward per side:
Dia change = 2 × 0.0005 = 0.001"
Pre-plate is same math as electroplate
using the outward growth value as t
Electroless Nickel on Threads — The Most Common Shop Problem
Electroless nickel (EN) is one of the most commonly specified coatings for aluminum and steel parts, and it causes the most thread fit problems when the allowance isn't calculated. A 0.001" per side EN coating adds 0.002" to every diameter — a bolt that was a perfect 2A fit before plating becomes an interference condition after. The correct solution is always to machine to the pre-plate allowance dimensions and verify with thread gauges before plating. Never try to "open up" a tapped hole after plating — the coating adhesion is excellent and reworking threads in a plated bore will strip the coating and expose base metal.
Coating Thickness Reference Table
Coating Process
Typical Thickness/Side
µm/Side
Dia Change
Thread Class Impact
Black Oxide
0.00005"
1–2 µm
~0.0001"
Negligible — no class change
Zinc Electroplate
0.00010"
2.5 µm
0.0002"
Minor — check 3A/3B threads
Cadmium Plate
0.00015"
4 µm
0.0003"
Minor — check 3A/3B threads
Copper Flash
0.00050"
12 µm
0.001"
Go one class looser before plate
Tin Plate
0.00050"
12 µm
0.001"
Go one class looser
Electroless Nickel (light)
0.00050"
12 µm
0.001"
Go one class looser
Electroless Nickel (med)
0.00100"
25 µm
0.002"
Go one class looser; verify after
Electroless Nickel (heavy)
0.00200"
50 µm
0.004"
Special allowance; consult plater
Hard Chrome (thin)
0.00050"
12 µm
0.001"
Go one class looser
Hard Chrome (medium)
0.00100"
25 µm
0.002"
Go one class looser; verify after
Anodize Type II
0.00010" outward
2.5 µm out
0.0002"
Check 3A/3B; usually minor
Hard Anodize Type III
0.00050–0.0010" out
12–25 µm out
0.001–0.002"
Go one class looser; always verify
Frequently Asked Questions
For electroless nickel, the deposition is very uniform — it's a chemical (autocatalytic) process that deposits the same thickness on all surfaces regardless of geometry, including inside thread roots and on thread flanks. This uniformity is why EN is specified for precision threads. Electrolytic processes (hard chrome, zinc, copper) do NOT deposit uniformly — high-current-density areas (thread crests, edges) receive more deposition than low-current-density areas (thread roots). Hard chrome can deposit 2–3× more on thread crests than in thread roots. For critical threaded applications, electroless nickel is strongly preferred over hard chrome when uniform thread coverage is required.
Generally no — and this is a common costly mistake. Hard anodize (Type III) creates a ceramic aluminum oxide layer that is extremely hard (60–70 HRC equivalent). Running a standard HSS tap through a hard-anodized hole will dull the tap rapidly and likely produce an undersized, rough thread. If you need to re-tap after hard anodize, use a carbide tap with appropriate speeds/feeds for hard aluminum oxide, or specify that threads be masked during anodizing (apply thread plug before tank immersion). The correct approach is always to calculate the pre-anodize allowance and machine to that allowance before anodizing — then verify with thread gauges after the process.
Use a Go/No-Go thread gauge set, but for the pre-plate condition you need gauges appropriate for the pre-plate tolerance class, not the final class. If you're machining a 2A thread pre-plate condition to allow for 0.001" coating, the pre-plate thread should pass the 1A Go gauge and fail the 1A No-Go gauge (or the low end of 2A). After plating, the finished thread should pass the 2A Go gauge and fail the 2A No-Go gauge. Many shops use thread ring gauges for external threads and thread plug gauges for internal threads. For high-volume plated fastener production, coordinate with your plating supplier to establish pre-plate gauge criteria based on their actual coating thickness capability, not just the nominal specification.
